The present invention relates generally to insulated gate field effect transistors (IGFETS) such as metal oxide semiconductor FETS (MOSFETS). More particularly, it relates to vertical MOSFETS such as vertical, double diffused (VDMOS) devices.
A conventional IGFET is a unipolar transistor in which current flows from a source region, through a channel in a body region, into a drain region. The source, channel and drain regions are of N or P type conductivity, and the body region is of opposite conductivity type. The channel is induced (in an enhancement type device) or removed (in a depletion type device) by means of an electrostatic field produced by charges on a nearby gate electrode. The gate electrode typically lies between source and drain electrodes (disposed respectively on the source and drain regions), and in a MOSFET device it is insulated from the semiconductor surface by an oxide layer.
In vertical MOSFETS the source and drain electrodes are on opposite semiconductor surfaces and create a current flow which is substantially vertical (perpendicular to the semiconductor surfaces) through the device. In VDMOS devices the gate electrode is typically on the same semiconductor surface as the source electrode, a configuration which provides a component of horizontal current flow (through the channel under the gate) as well. However, the transition from horizontal to vertical current flow produces current crowding, an effect which impedes device performance by reducing the maximum achievable voltage gain.
Additionally, in conventional VDMOS structures the gate typically overlaps that portion of the drain region adjacent to the channel, producing a capacitance C.sub.GD. When multiplied by the device gain ##EQU1## this capacitance is referred to as Miller-feedback capacitance, and it impairs device operation at high speeds and voltages.
To provide higher frequency and higher voltage operation, the present invention introduces a structure which reduces both Miller-feedback capacitance and current crowding in vertical MOSFET devices.